A well known method of controlling the voltage to, and thus the speed, of a battery-driven DC motor uses a rapidacting switch, called a chopper, in series with the motor. Such a chopper type motor drive may accomplish speed control by pulse width modulation to vary the average power input to the DC motor, i.e., by switching a solid state power device such as a silicon controlled rectifier (SCR) between the "on" state and the "off" state and controlling the percentage of "on" time. Other known chopper drives vary the frequency of constant width pulses to vary the average power to the motor. Many known chopper circuits interrupt the current through the load current carrying SCR by "forced commutation," the essence of which is to decrease the load current through the SCR to a value below the "hold-in" current for an interval greater than the "recovery period" of the SCR. Capacitor-type forced commutation chopper circuits charge a commutation capacitor in shunt to the main load current carrying SCR during the "on" time and gate a commutation SCR in the shunt path to discharge the commutation capacitor in the reverse direction through the load current carrying SCR to cause it to commutate off.
The current flowing through the tractive motor of known forced commutation chopper controls may become excessive during such conditions as start-up, acceleration, plugging or when the vehicle starts up an inclined ramp, and such excessive current may result in failure and accelerated wear of the motor brushes as well as overheating and eventual failure of the motor.